Inverters are used in energy generation systems, for example photovoltaic systems, called PV systems for short below, to convert a direct current into alternating current which can be fed in a single-phase or multiphase manner into a public or private energy supply network. In the case of a photovoltaic system, the direct current to be converted is provided by a photovoltaic generator (PV generator) which comprises any arrangement of a plurality of photovoltaic modules (PV modules) within the scope of the application. Alternatively and/or additionally, (possibly buffered) direct current can be provided from batteries or fuel cells or comparable chemically operating current sources.
Such energy supply systems have an arrangement of (buffer) capacitors in a DC intermediate circuit in order to smooth the DC voltage provided by the DC generator during conversion into alternating current. In the case of a single-stage inverter in which the voltage from the DC generator is immediately applied to the input side of an inverter bridge, such a capacitor arrangement is connected in parallel with the DC generator. In the case of a multistage inverter, at least one DC-DC converter is connected upstream of the inverter bridge and steps up or steps down the DC voltage provided by the DC generator to a voltage level suitable for operating the inverter bridge. In the case of such a system, the capacitor arrangement is usually arranged between the DC-DC converter and the inverter bridge. Within the scope of the application, the circuit in which the capacitor arrangement is arranged is referred to below in a generalized manner as the intermediate circuit, in a manner following the conventional terminology, independently of the position of the capacitor arrangement. Accordingly, the capacitor arrangement used to smooth the voltage is referred to as the intermediate circuit capacitor arrangement in both cases.
The inverter bridge of an inverter is usually fitted with power semiconductor switches which are operated in a clocked manner in a modulation method. A known and conventional modulation method is the pulse-width modulation method (PWM method) which is operated at a clock frequency, that is to say a number of switching cycles per second, in the kilohertz range. As a result, a clocked DC signal whose polarity changes is provided at the output of the inverter bridge, which DC signal is smoothed by an output current filter in such a manner that a voltage profile which is as sinusoidal as possible is produced at the output of the filter. For this reason, the output current filter is often also referred to as a sinusoidal filter.
In this case, the output current filter comprises a plurality of inductances and capacitances. A known output current filter which is frequently used has at least one inductance for each of the phases of the inverter, for example a coil which is arranged between the respective output of the inverter bridge and the corresponding phase connection of the energy supply network. Furthermore, a capacitance is respectively arranged between each output connection of the inverter bridge and a neutral conductor of the system at the inverter-side input of the filter. On the output side, i.e. toward the energy supply network, a second capacitance is provided for each phase. This second capacitance is connected in a star connection, i.e. it makes contact with a common floating voltage node. The first and second capacitances are usually formed by corresponding first and second capacitors.
The correct and reliable function of the inverter of the energy generation system is largely dependent on the capacitances mentioned, the intermediate circuit capacitance and the capacitances in the output current filter. However, the capacitors used to provide the capacitances are subject to ageing processes which reduce their capacitance value over time. In the case of electrolytic capacitors, a strong temperature dependence is additionally observed at temperatures below the freezing point. With knowledge of the changed capacitance values, the correct method of operation of the inverter can be corrected, up to a certain capacitance loss, by adjusting parameters of the inverter, for example regulating parameters which determine the switching times in the switching cycle of the power semiconductors of the inverter bridge. In the case of excessively large deviations of the capacitances, it is useful to terminate the operation of the inverter in order to avoid more far-reaching destruction of the inverter or the capacitors. Knowledge of the capacitance values of capacitances of the energy generation system, in particular of the intermediate circuit capacitances and the filter capacitances, is desirable both for adjusting the operating parameters of the inverter and for disconnecting the inverter or for outputting a warning indicating in advance problems which can possibly be expected.
In this respect, the document DE 10 2004 036 211 A1 discloses, for example, a method in which an intermediate circuit capacitor is pre-charged via a charging resistor during activation of the apparatus. The capacitance of the intermediate circuit capacitor can be determined during pre-charging from a measurement of the charging current and a measured voltage profile at the intermediate circuit capacitor. This method is suitable, in particular, when pre-charging of the intermediate circuit capacitor(s) is provided and the apparatus has a corresponding pre-charging apparatus.
In a similar manner, the intermediate circuit capacitor of an inverter in an engine controller is discharged by means of a discharging resistor during inactivity of the engine controller according to the document WO 02/18962 A1. The capacitance of the intermediate circuit capacitor is determined from a measured voltage profile during discharging.
The document US 2012/0281443 A1 discloses a method for determining a defective capacitor in an intermediate circuit consisting of a series circuit of a plurality of capacitors. In this case, the voltages dropping across the individual capacitors are measured and a defective capacitor is inferred from the level of the voltages. The document US 2013/0155729 A1 describes a method for predicting an expected service life of an intermediate circuit capacitor in a motor converter. In this method, an AC component flowing in the capacitor is determined and is used to determine a power deposited in the capacitor during operation. The deposited power is used to infer the ageing state and therefore the service life of the capacitor which can still be expected. However, capacitance values of the capacitors cannot be determined using the methods described in the two documents mentioned.
The document US 2009/0072982 A1 describes a system for energy conversion in which the temporal variation of the voltages across capacitors in the system is measured and the level of a voltage ripple at the capacitors is determined. In addition, the currents flowing during the occurrence of the voltage ripple are determined. The capacitance of the capacitor is determined from the level of the voltage ripple and the level of the flowing current. The document EP 2 690 452 A2 also describes a comparable method. These methods may be advantageous if it is desirable to determine the capacitances during normal operation of the energy supply system. However, for reasons of safety, it is often desirable to accordingly diagnose the correct functionality of the capacitors before an energy generation system is connected to the energy supply network. For example, it may be problematic to operate an energy supply system at high power if the capacitance of intermediate circuit capacitors has fallen to extremely low values owing to the temperature. This problem occurs, for example, in connection with electrolytic capacitors as intermediate circuit capacitors in ground-mounted PV systems under extreme weather conditions. Such a PV system is generally started only after the intermediate circuit capacitors have been heated by means of a heating apparatus provided for this purpose.